Friday, July 27, 2012

It only took about 300 million years ...

... for the forces of natural selection to produce spiders. Silk-producing arachnids go back even farther (nearly 390 million years), and chelicerates – distant cousins of spiders – were among the first animals to leave the sea and conquer dry land. Of the roughly 40,000 species of spiders in the world about 5,000 belong to the family Salticidae, the jumping spiders.

Jumpers are fun spiders to watch. Unlike the orb weavers which simply hang in the middle of a web waiting for a meal to come to them, jumping spiders actively hunt for their dinner – they almost seem to have character. The sturdy female spider in the next two pictures is a Phidippus whitmani. A photo of a male, which is a striking bright red rather than brown, would have brightened up the color scheme.

The average Phidippus clarus in my area also tends to be a rusty brown but this summer I was lucky and stumbled across several males that were a beautiful red. It's tough to get a dorsal shot of that abdomen, as jumping spiders have a habit of facing toward the camera, aiming those arresting anterior median eyes at whatever they perceive to be a threat. See what I mean about character?

Phidippus clarus is a good sized jumping spider about 10 mm long, my finger to the left in the photo gives a sense of scale.

A male Marpissa formosa basking on the rocks along the river shoreline, also about 10 mm in length but elongated and lean rather than robust.

I was at unable to identify the male spider in the following three images. Despite its length of 6 mm and the shape of the carapace being completely wrong, I nevertheless – tentatively, reluctantly, unhappily – called it a "crab spider" (male crab spiders are tiny, much smaller than 6 mm). This diagnosis was less than satisfactory so I submitted the images to the always helpful people at BugGuide.Net, and it turns out that my "crab spider" is an old friend ...

... Araneus trifolium, the Shamrock Orbweaver. (The little red hitchhiker on the spider's abdomen is a parasitic larval mite, family Erythraeidae.) As is often the case with spiders the male is much smaller than the female, the old girl in the photo below is a good 25 mm in length, and gravid, ready to lay eggs any time. Talk about sexual dimorphism ... looking at these two spiders, who would guess that they were the same species?

This is a white form female, and if the difference between the male and female aren't enough to cause confusion, female Shamrock Orbweavers, a.k.a Pumpkin Spiders, come in an awesome array of colors!

Whilst they don't come in a baffling variety of tints and hues, Flower Crab Spiders such as Misumena vatia can change their color from yellow to white in order to match the color of the flower they are lurking on. Obviously the Flower Scarab (genus Trichiotinus, itself an accomplished bee mimic) is unaware of how perilously close it is to its possible demise ...

Although many species of wasps are parasitoids of spiders, the Thread-waisted Wasp (Ammophila sp.) provisions its larvae with caterpillars and this wasp was simply stopping at these False Solomon's Seal blossoms for drink of nectar, not hunting the spider as a potential source of nourishment for its young.

But Flower Crab Spiders blend into the background so well they are virtually invisible, nature seldom forgives errors in judgement, and unlike the Flower Scarab this wasp wasn't so lucky. The Thread-waisted Wasp won't be passing on its genes to the next generation, the Flower Crab Spider might – if it survives the hazards and vicissitudes of an uncertain life – and who knows ... her descendants may still be around in another 300 million years.